This invention relates to a magnetic resonance imaging (MRI) system in which an excitation pulse, a high frequency magnetic field or an RF (radio frequency) field, is applied to an object being examined, located in a static field and applied with a suitable gradient flied, to excite a magnetic resonance (MR) phenomenon, and MR signals generated by a specific atomic nucleus spin in the object is received, and appropriately processed, to obtain the image information at a selected portion of the object. More particularly, this invention relates to a system which can reduce the time taken for the MRI system adjustment.
In the MRI system, a uniform static magnetic field is applied to a desired portion of the object. The same portion is applied with a suitable gradient field and an excitation pulse, an RF magnetic field, orthogonal to the static magnetic field. Under this condition, the MR phenomenon occurs in the selected slice portion of the object. An MR signal is generated by a specific atomic nucleus spin in the object through the MR phenomenon exactly after the RF magnetic field is removed. The MR signal is received and appropriately processed. Finally, the image information of the selected portion of the object is obtained. Two independent RF coils or a single RF coil for both transmission and reception is used for transmitting the excitation pulse and receiving the MR signal. The gradient fields usually used are a gradient field to determine the slice position with respect to the static field direction and a gradient field for the phase encoding and the reading of the MR signal used when the two-dimensional Fourier conversion method is employed.
In the MRI system, before two steps of operations, the MR exciting and the collection of the MR signal, are performed for obtaining the MR image, the same operations are performed in a condition that the object is placed in a predetermined image pick-up area. These operations are needed for adjusting the various parameters of the MRI system.
The automatic adjustment of these parameters is called a "prescan". The parameters subjected to the prescan contain mainly a static field intensity, a tuning for receiving an MR signal, a transmitting power, and a receiving gain in the receiving system.
The adjustment of the static field intensity is normally performed when the normal conductivity coil is used for generating a static magnetic field. This adjustment optimizes the magnetic field intensity. An improper static magnetic field causes a beat in the MR signal, and reduces the level of the received MR signal. The improperty of the static magnetic field arises mainly from a variation in the power voltage for driving the static field coil, which is for generating the static magnetic field. The static magnetic field adjustment is such that the MR signal received is Fourier analyzed to detect the beat, and some adjustment is made to remove the beat.
Due to the tuning for receiving an MR signal a receiving system in the MRI system is tuned to the MR signal. The detuning of the receiving system occurs due to the fact that the resonating conditions of the receiving system depends on the size of the object under examination. The untuned receiving system provides a reduced level of the received MR signal. The tuning is the adjustment of a tuning parameter of the tuning circuit, for example, a capacitance of a tuning capacitor, in the receiving system, such that the received MR signal level is maximized.
The transmitting power adjustment is for optimizing the power of the exciting pulse, e.g., a .pi./2 (90.degree.) pulse and/or a .pi. pulse (180.degree.) respectively for flipping the magnetization by 90.degree. and by 180.degree.. The power of the excitation pulse necessary for flipping the magnetization by a predetermined angle is large for large objects, and small for small objects. The exciting pulse adjustment optimizes an accuracy of the flip angle of the magnetization by the exciting pulse. An inappropriate flip angle provides a level of the received MR signal. To avoid this, the exciting pulse power is adjusted so as to maximize the magnitude of the received MR signal.
The receiving gain adjustment is for adjusting a gain of the receiving system. The received signals are not uniform in level. The gain in the receiving system is adjusted so that the received signal levels fall within the dynamic range of the input stage of a signal processing system. This adjustment is made for ensuring the satisfactory operation of the signal processing system.
These adjustments must be made every time the object is exchanged by a new one.
FIG. 1 shows a sequence of the automatic adjustment process on the basis of the prescan including the adjustments of the static field, tuning system, transmitting power, and receiving gain, when it is executed by a conventional MRI system. In the figure, A1 to Ana, B1 to Bnb, C1 to Cnc, and D1 to Dnd represent the excitations for those adjustments of the static field, tuning system, transmitting power, and receiving gain. The number of those excitations na, nb, nc and nd have different figures. For ease of explanation, the average value of these figures is used and denoted as n. Each excitation is performed at repetition periods TR. As seen from FIG. 1, the above four adjustment-required parameters are adjusted in a time-sequential manner. A total time TT required for those four parameters is expressed by the following equation ##EQU1## If n=16 and TR=1 sec., TT is 64 sec.
As already known, the image pick-up time in the MRI system is longer than that of other tomography systems, e.g., an X-ray CT (computed tomography). A long prescan time hinders the quick operation of the image pick-up, and places a patient, if the object is the patient, under uneasiness. For these reasons, it is desirous to reduce the prescan time as short as possible.
The reduction of the prescan time is the technical problem now being encountered and urgently solved in this field.